Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates to electrical components which generate heat during use and, more particularly, to methods and apparatuses for dissipating heat from such electrical components.
2. Description of the Background
Electrical components, including electrical semiconductor components, are used in numerous electrical devices in all sorts of applications. Many electrical components generate heat during use due to the electrical current flow therethrough. If not effectively dissipated, this generated heat can be detrimental to the performance of the electrical component and to nearby structure or components, both in terms of electrical performance and in terms of mechanical integrity.
Many methods are used to prevent the excessive build-up of heat or temperature. Considerable heat may be dissipated through convection to the gas surrounding the electrical component, such as by inducing a forced flow of air across the electrical component. However, inducing air flow requires adequate space such as for a fan and for air flow channels, and also increases cost. Some components are housed in a sealed compartment, which may for instance be filled with nitrogen, rendering forced gas flow impossible. Many components require a higher rate of heat dissipation than can be provided by mere convection.
Accordingly, it is common to increase heat dissipation by attaching a heat sink to a face of the electrical component. The heat sink is typically formed of metal, although other highly thermally conductive materials may also be used. Heat is conducted from the electrical component to the heat sink and then conducted along the heat sink away from the electrical component and dissipated from the surfaces of the heat sink. The heat sink may have a considerable thermal mass relative to the electrical component, absorbing a considerable amount of the heat initially generated by the electrical component. The heat sink typically has a larger surface area than the electrical component. Because heat dissipated by convection is a function of the surface area exposed to the convecting fluid, heat convects from the heat sink much more rapidly than from the electrical component.
While the heat sink itself may function very well, attachment of the heat sink to the electrical component poses problems. The heat sink must make sufficient thermal contact with the electrical component over a significant surface area, so heat will adequately conduct from the electrical component to the heat sink. Providing sufficient thermal contact may be difficult, particularly over a long duration of the life of the electrical component. Providing sufficient thermal contact is more difficult if the attachment is under repeated mechanical stress, such as when subject to vibration, or if the attachment is subject to shock such as if the electrical device is inadvertently dropped. Providing sufficient thermal contact is also more difficult when the attachment repeatedly undergoes thermal stress due to changes between environmental conditions as well as between operating and non-operating conditions. The severity of the thermal stress is a function both of the magnitude of the temperature change and the rate of temperature change witnessed by the component. The attachment mechanism must ensure intimate physical contact over all operating and use conditions.
Each electrical component typically includes two or more electrical leads that must be electrically insulated one from the other. The heat sink must not provide an electrically conductive path between such leads, or even pose a risk of such a short circuit. In many applications and particularly in power environments, the heat sink must prevent any risk of an electrically conductive path or short circuit from each electrical component to other components in the electrical device. Heat sinks may be formed of electrically insulative or dielectric materials to prevent such a short, but most electrically insulative materials are also thermally insulative, and the heat sink must be thermally conductive to effectively increase heat dissipation. Instead, the attachment between the heat sink and the electrical component may be electrically insulative, and the heat sink formed of metal.
The electrical component may undergo a significant temperature difference from a cold or room temperature when not being used to a high temperature steady state operating condition. If the electrical component is used outdoors, the environment of use of the electrical component may also include temperature and humidity extremes, such as from sub-zero temperatures to desert sun and heat. The heat sink is typically formed of a different material than the electrical component, and thus typically has a coefficient of thermal expansion which is different than the coefficient of thermal expansion of the electrical component. The attachment between the electrical component needs to accommodate the differing thermal expansion rates over repeated cycling between hot and cold temperatures.
The size of any attachment means between the electrical component and the heat sink should be minimized, so as to reduce the size of the electrical component/heat sink product. Many methods of attachment require significant additional space beyond that required just by the electrical component and the heat sink. A certain safety clearance may be required between neighboring electrical components, which may further compromise space requirements if the connection means between the heat sink and the electrical component adds additional size. To reduce the total cost of the electrical component, the process of assembling the heat sink to the electrical component should be as simple as possible. Many methods and/or structures of attachment are not easily automated, increasing the final cost of the electrical component/heat sink product.
Many heat sinks are attached to electrical components with mechanical fasteners, such as one or more bolts, clips or clamps. The mechanical fasteners take up space, which is significant in the environment that many electrical components are used. To electrically insulate the heat sink from the electrical component(s), an additional electrically insulating structure or material must be placed in the interface between the heat sink and the electrical component(s). For instance, it is known to place a material in the interface between the heat sink and the electrical component, which are then attached together with bolts. One such material, known as BOND-PLY(copyright), a registered trademark of Bergquist Company, Minneapolis, Minn., includes a layer of fiber glass cloth between layers of precured silicone. The precured silicone is then placed against the heat sink prior to mounting the heat sink to the electrical component. Polyester sheet materials have also been used between the heat sink and the electrical component. In general, polyesters are too thermally insulative, and must be applied in too thin a layer to be usable. That is, to provide adequately low thermal resistance, the polyester sheet must be at a thickness such as about one mil, which renders the material too weak and easily punctured. As another example, a cured silicone-based laminate on a KAPTON(copyright) film structure material, a registered trademark of E. I. DuPont de Nemours and Co., Wilmington, Del. is available in sheet form of about 0.0025 inch thick in a K-10 XT grade which has high tensile strength and good puncture resistance. However, the KAPTON(copyright) film material is expensive and significantly increases the cost of the electrical component/heat sink combination.
To ensure intimate physical contact between the electrical component, the electrically insulating structure and the heat sink, bolts must be properly torqued, and clips must be designed and attached to provide a proper compression force. Provided a proper torque or compression force is applied to the bolt/clip(s), the positive structural connection provided by most bolt/clip(s) will adequately accommodate the differing thermal expansion rates between the electrical component and the heat sink.
If formed of an electrically conductive material such as metal, the bolt/clip(s) may introduce an undesired potential electrical flow path between the heat sink and the electrical component, despite the presence of the electrically insulating structure. Alternatively, additional size may be required to adequately space the bolt/clip(s) relative to the electrical component. The bolt/clip(s) and electrically insulating structure require additional cost for manufacture and stocking. Assembly with the bolt/clip(s) and insulating structure is difficult to automate and further increases cost.
As an alternative to mechanical fasteners, adhesives have been used to connect an electrical component to a heat sink, such as a silicone based adhesive or an epoxy based or urethane adhesive such, or an acrylic based adhesive. The adhesive is applied in a liquid or paste condition to the interface between the electrical component and the heat sink. Many such adhesives come in two parts which require mixing immediately prior to application. The electrical component and the heat sink are brought into contact with the adhesive in the interface, and the adhesive is then cured or solidified by time, heat, and/or ultraviolet or infrared radiation. The relevant art also teaches using such an epoxy to bond an insulated printed circuit board having many electrical components connected thereto to a heat sink.
To be effective for the heat sink, the adhesive has to provide minimal thermal resistance, which is generally addressed by having a minimal thickness and/or by adding a thermal carrier. To be effective particularly in a power environment, the adhesive has to also be electrically insulative, which significantly limits the types of adhesives that can used. For instance, the adhesive may be an electrically resistive base material loaded with a thermally conductive carrier such as a boron nitride or aluminum oxide filler. Additionally, the added cost of the adhesive substance and its application should be minimized.
Adhesive attachments also have proven problematic. Care must be taken to sufficiently xe2x80x9cwet-outxe2x80x9d the adhesive, to provide intimate physical contact and minimize the resistance to thermal conduction at the interface. The desire to provide a sufficiently thick layer to ensure adequate wetting-out runs counter to the desire for the adhesive layer to be as thin as possible, and consistent application at the same thickness is difficult. The degree of physical and thermal contact may worsen as many adhesives degrade over time or through thermal cycling and become less flexible or more brittle. Aging of the cured adhesive may also detrimentally affect the thermal conductivity through the adhesive. The problem of providing intimate physical contact may worsen particularly due to the shear induced by thermal cycling caused by differing thermal expansion rates. Application of the adhesive is messy and difficult to automate. Depending on the type of adhesive, and particularly if the adhesive involves a reaction between two or more substances, the adhesive may produce out-gassing which is detrimental to the electrical component.
Accordingly, there exists a need in the relevant art for a method to attach electrical components to a heat sink in a cost-effective manner without sacrificing the performance of the components. There further exists a need in the relevant art for a highly automated method for attaching electrical components to a heat sink, and which uses less expensive raw materials in comparison to the relevant art. There also exists a need in the relevant art for a method to attach electrical components to a heat sink in a space-effective manner without sacrificing the performance of the components.
The present invention is directed to a method for fabricating an electrical apparatus. The method includes providing a planar element including a first electrically and thermally conductive region and a second electrically and thermally conductive region, such that the first and second regions define a spacing therebetween, and wherein the planar element includes at least one mechanically stabilizing tie connected between the first and second regions across the spacing, directly connecting a first terminal of an electrical component to the first region, directly connecting a second terminal of the electrical component to the second regions, such that the electrical component bridges the spacing, and removing the at least one mechanically stabilizing tie from between the first and second regions.
According to another embodiment, the invention includes providing a planar element including a first electrically and thermally conductive region and a second electrically and thermally conductive region, such that the first and second regions define a spacing therebetween, and wherein the planar element includes at least one mechanically stabilizing tie connected between the first and second regions across the spacing, directly connecting a first terminal of an electrical component to the first region, directly connecting a second terminal of the electrical component to the second region, such that the electrical component bridges the spacing, removing the at least one mechanically stabilizing tie from between the first and second regions, connecting a heat sink in thermal communication to the first and second regions, and electrically insulating the heat sink from the first and second regions.
According to another embodiment, the present invention is directed to an electrical apparatus. The electrical apparatus includes a first electrically and thermally conductive region, a second electrically and thermally conductive region, wherein the first and second regions define a spacing therebetween, an electrical component having a first terminal directly connected to the first region and a second terminal directly connected to the second region, wherein the electrical component bridges the spacing between the first and second regions, a heat sink connected in thermal communication to the first and second regions, and a silicone bonding sheet cured between the heat sink and the first and second regions.
The present invention represents an advancement over the relevant art in that it provides a method of attaching electrical components to a heat dissipative device, such as a heat sink, through lower costing raw materials and lower labor costs, without sacrificing the performance of the electrical components. The present invention also represents an advancement over the relevant art in that it provides a space-effective manner of attaching electrical components to a heat sink. These and other advantages and benefits of the present invention will become apparent from the Detailed Description of the Invention hereinbelow.